Tuesday, January 06, 2009

With the possible exception of the ice that covers Greenland, the West Antarctic ice shelf is the most important body of water in the world. If it thaws, the results will be disastrous for millions, raising sea levels and flooding coastal cities such as London, New York, Tokyo and Calcutta. So it is understandable that scientists are alarmed as to why one particular section of it - Pine Island Glacier - is melting so much faster than the rest.

Pine Island, which contains around 30 trillion litres of water, is slipping into the sea at an ever accelerating rate, a development that alone could raise sea levels by as much as 10cm over the next century. Starting at an altitude of 2,500m, the glacier is 95 miles long and 18 miles wide, reaching the sea as an ice wall 750m high. Even before it began to speed up, it was one of the fastest-flowing glaciers in the world, at nine yards a day.

Scientists believe that the thinning of the glacier, and its acceleration, are due to unusual melting under the base as it enters the ocean. This is caused by either global warming or a hitherto unknown factor, such as an underwater volcano.

Finding proof of either, however, has been problematic. The mountain glaciers in the west of the Antarctic have the worst blizzards and some of the harshest temperatures on the planet. The zone is too hostile for any research station, so scientists have to base information on satellite studies and aerial surveys.

Now, though, a team of British scientists plans to attack the continent's "weak underbelly" – the watery realms below the massive glaciers. Using a robot submarine nicknamed "Autosub", researchers from the British Antarctic Survey (BAS) hope to explore this previously inaccessible part of the glacier, to discover what is happening.

The unmanned submarine, seven metres long and painted bright yellow, will use sophisticated sonar scanners to map out the underside of the ice, while measuring changes in water temperature, pressure and salt content beneath the glacier.

"We believe that something about the ocean around where the glacier ice moves from being grounded to floating has changed, and this is driving the thinning and accelerating ice flow," explains Dr Adrian Jenkins, who is leading the BAS research. "But we really have very little idea of what is actually going on beneath the ice, as we have not been able to see through it – it is more than a kilometre thick in places. The only way is to send in our instruments to get measurements."

The submarine, which has a maximum range of 248 miles and is powered by 5,000 ordinary D-cell batteries, will navigate its way for 40 miles underneath the glacier ice, until it reaches the point where the ice meets the land – a journey that will take 20 hours.

Using its sonar, the Autosub will pick its way through the water, while creating a three-dimensional map that the scientists can later use to determine the areas most prone to melting.

The submarine will follow a pre-programmed course, but can find its way around any obstacles.

The mission is risky – a previous Autosub was lost as it was exploring the Fimbul ice shelf in another part of Antarctica, while another mission in 2003 had to be abandoned due to poor weather. But its importance is clear.

"In the space of just two seasons, the ice was moving 6.4 per cent faster, which is exceptional," says Dr Julian Scott of the BAS, who placed GPS trackers on the ice to measure its movement.

"The glacier is out of balance, as more ice is being lost to the sea than can be replaced by snow falling. This imbalance meansthe glacier is contributing more and more to the sea level rise."

One explanation is that the acceleration is the result of changes in the temperatures underneath. Warm water from the North Atlantic is driven into the southern oceans by strong currents, where it sits as a dense layer beneath the much colder surface water.

The warmer water occasionally spills on to the high seabed shelf that stretches out from the Antarctic landmass. The scientists believe that deep grooves in the seabed have been channelling warm water underneath the glacier, where it melts the ice.

"This is the only area of West Antarctica that is changing really rapidly," says Dr David Vaughan, an expert in glacial retreat at the BAS.

"It is vital that we understand why this is happening so we can predict how the ice will change in the future and how much it will contribute to sea level change."